This invention relates to heaters and in particular space heaters of the type intended to heat large buildings such as warehouses, factories, hangars etcetera.
Space heaters are known from inter alia, WO-A-96/10720, GB-A-2 145 218, EP-A-0 408 396, EP-A-0 408 397, and EP-A-0 410 707. The heaters disclosed in EP-A-0 408 396 comprise a housing, an elongate combustion tube, a burner at one end of the tube and a fan at the other end of the tube for drawing combustion gases through the tube. The housing is open at its lower end to permit radiant heat from the combustion tube to be directed downwardly from the heater towards the ground. In addition to providing radiant heating, the heater disclosed in EP-A-0 408 396 is also arranged to provide blown warm air heating. This is achieved by providing an air duct which passes along the interior of the housing such that the air inside is warmed as it moves along the air duct, apertures in the air duct allowing the warm air to escape in a downwards direction. In addition, at the air inlet a proportion of the air can be diverted by deflector plates such that the air impinges directly onto the combustion tube thereby providing a faster rate of heating of the air before it passes out through the bottom of the heater towards the ground. The positions of the deflector plates can be varied so as to divert more or less air directly on to the combustion tube so as to achieve a desired balance between the radiant heat and the convected heat emitted by the heater.
One consequence of directing air on to the combustion tube directly is that it cools the tube which can lead to loss of radiant efficiency and can also cause the formation of soot and condensation within the combustion tube. A further potential disadvantage of the heater disclosed in EP-A-0 408 396 is that there is only a single centrifugal fan at one end of the heater and hence it is difficult to ensure that the output of heated fan-assisted convected air from the heater remains at a useful and constant level along the length of the heater; particularly in heaters of longer length.